The mechanism by which skin pigmentation is formed, namely by which melanins are formed, is particularly complex and schematically involves the following main steps:
Tyrosine.fwdarw.Dopa.fwdarw.Dopaquinone.fwdarw.Dopachrome.fwdarw.Melanins, PA1 r' and r", identical or different, represent a hydrogen atom, a C.sub.1 -C.sub.20 alkyl radical, a C.sub.2 -C.sub.6 hydroxyalkyl radical, or a C.sub.3 -C.sub.6 polyhydroxyalkyl radical, or r' and r" taken together, with the nitrogen atom, form a heterocycle; PA1 the number of carbon atoms in the --(CH.sub.2).sub.n --COR.sub.1 chain is less than or equal to 21; PA1 R.sub.2 and R.sub.3, identical or different, represent a hydrogen atom, a linear or branched C.sub.1 -C.sub.4 alkyl radical, or a C.sub.1 -C.sub.4 alkoxy; PA1 R.sub.4 represents a hydrogen atom or a linear or branched C.sub.1 -C.sub.4 alkyl radical; and PA1 n is an integer from 0 to 20, provided that, when R.sub.2 and R.sub.3 represent a hydrogen atom, n is greater than or equal to 2.
with the enzyme involved in this series of reactions being essentially tyrosinase.
The substances in widest use at the present time as depigmentors are in particular hydroquinone and its derivatives, particularly its ethers such as hydroquinone monomethyl ether.
These compounds, while they are definitely effective, are unfortunately not bereft of side effects, which can make their use delicate or even dangerous.
Thus, hydroquinone, whose use is moreover limited to a concentration of 2%, is a compound that is particularly irritating and cytotoxic to the melanocyte, and whose total or partial replacement has been considered by many authors.
Thus, U.S. Pat. No. 4,526,779 has proposed certain hydroquinone fatty esters that have good activity and are less irritating and more stable than hydroquinone.
Likewise, other hydroquinone derivatives that do not have the drawbacks of hydroquinone but whose efficacy has proved relatively poor have been proposed in Japanese Patent Application No. 27909/86.
It is well established that a substance has a depigmenting effect if it acts directly on the vitality of the epidermal melanocytes where melanogenesis normally occurs and/or if it interferes with one of the stages in melanin biosynthesis, either by inhibiting one of the enzymes involved or by intercalation as a structural analog in the synthesis pathway which can accordingly be blocked, hence the depigmenting effect.
The use of topical depigmentors that have good efficacy and are harmless is particularly desirable with a view to treating regional hyperpigmentation caused by melanocytic hyperactivity such as idiopathic melasma occurring during pregnancy (mask of pregnancy or chloasma) or secondary to estrogen-progesterone contraception; local hyperpigmentation caused by benign melanocytic hyperactivity and proliferation such as lentigo senilis, or liver spots; accidental hyperpigmentation such as postlesional photosensitization and scarring; and certain forms of leukoderma such as vitiligo where, if the injured skin cannot be repigmented, the residual zones of normal skin are depigmented to impart a homogeneous white color to the entire skin.